The present invention relates to a method and apparatus for fluid compression of flowable plastic material following injection of the plastic into a mold cavity whereby to form a solid injection molded part having no internal voids, that is substantially strain free, and has a Class A finish and sink-free surface.
Injection molds typically comprise stationary and moving mold halves (i.e., the core side and cavity side) which are closed and clamped together to form a mold cavity therebetween for shaping articles from thermoplastic compositions. The thermoplastic is heated into a molten condition and injected under pressure through a nozzle and into the mold cavity by means of a screw ram. Injection pressures of 2,000 to 10,000 psi are common at the gate locations. The plastic is allowed to cool to sufficiently harden the thermoplastic whereupon the mold is opened and the hardened articles removed.
A traditional plastic molding problem is the formation of surface distortions or "sink marks" on the appearance side caused by ribs or bosses on the backside of a part resulting from the volume contraction (i.e., shrinkage) of the plastic during cooling. Further, warpage or part distortion can result from the high injection pressures used to fill the cavity, the pack out pressure, or from an uneven pressure gradient resulting from the injection pressure at the gate being higher than the pressures at the extreme ends of the molding. High injection pressures can cause strain marks or molded in strain in the hardened article, resulting in warpage at once, or over a period of time after molding, or if the end use of the molding is in a high temperature area. When ribs are formed in the molding, due to a shrinkage differential, the wall thickness versus rib configuration can cause the ribs to buckle or bend the molding. In large projected area moldings where the plastic cannot flow from the gate to the end of the molding, hot runner molds are needed and high clamping forces (e.g., 1,000 to 5,000 tons) are required to hold the mold halves together. These molds are costlier to build, and the runners can add a weld line to the product. Machines which can provide these high clamping forces are costly to operate.
In what has come to be known as "gas assisted injection molding" an inert gas is injected through the plastic injection nozzle and directly into the thick areas of the melted thermoplastic, whereby to create hollow sections in the part. With the gas assisted molding process, sink marks and warpage can be minimized, and possibly eliminated. The gas is directed through a hollow (i.e., gas channel) of the material formed between the surface of the part and a backside detail, such as a rib. If so, the base of the rib must be made thicker to help direct the gas channel, which is just the opposite of normal design practice with plastic where ribs are made as thin as possible to try and eliminate shrinkage. With the gas channel at the base of a rib, material will shrink away from the inside surface of the channel as the molded part cools because the material is the hottest at the center of the section. Therefore, as the plastic part shrinks during cooling, the sink mark on the visible outside surface is minimized.
A disadvantage in such gas assisted molding operations is that the gas pressure within the channels must be released prior to opening the mold, which normally requires costly post molding steps of venting the pressurized gas to atmosphere and then sealing or finishing this hole. Oftentimes sealing of this vent hole is needed, such as where the appearance or function of the part is affected, or to obviate the possibility of the part contaminating various chemical baths during secondary operations, such as chrome plating or painting.
Additionally, the possibility of achieving a Class A surface is inhibited by shadow marks caused by gas holes in the thicker areas of the molding, and gas permeation caused by the gas not being retained in the thicker areas and overflowing into the wall thickness of the molding. This causes thinning and weakening of the wall, raised areas, and blush marks.
In the gas assisted process, the gas used during the molding operation can be recovered to some extent but the chances are it will be full of volatiles from the molded polymer which would need to be removed. However, there are dangers in compressing inert gas with a volatile gas (e.g., fire).
Additionally, with gas assistance, costly apparatus is needed in the form of gas compression units, nozzles, pins and the like to introduce the gas into the molding. Further, to operate these units at the high pressures needed (e.g., 9,000 psi) is energy costly, the gas used and lost is costly, and the cost of maintenance is high.
Injection molding of parts utilizing a pressurized gas source is shown in "Injection Mold Method and Apparatus," published 14 Jun. 1990 as PCT Publication WO 90/06220, the specification being specifically incorporated herein by reference. While this process is suitable for molding articles of the type shown therein, there is always a need for improvements in forming low cost articles.
The primary objects of this invention are to provide a method and apparatus which enhances the low cost production of a plastic molded part which is stress-free, has a Class A surface condition, is free of "sink-marks" or "blush-marks," has no gas internally in the part or voids internally of the plastic, avoids permeation and witness lines, does not require venting the fluid pressure within the molded part, provides a constant gas pressure across an inner surface of molten plastic used to form the mold part, and allows for the reclaiming of the fluid (i.e., gas) with reduced volatile content for reuse in the process.
Yet another object of this invention is provision of mold apparatus which eliminates the need for gas channels to communicate gas to remote locations whereby to form free-standing bosses, stiffeners, and other structural details.
A further object of this invention is provision of an injection molded, gas compressed, dimensionally stable, thermoplastic part having reduced wall thicknesses, without the need for either reinforcement ribs, as desired, or internal gas cavities.
Yet another object of this invention is provision of a process that is efficient, requires lesser pressure to form a part, reduces the clamping forces needed to retain the molds together against the pressure, obviates venting, and advantageously uses at least part of the forming pressure to assist in ejection of the finished part upon opening of the mold portions.
A further object of this invention is provision of a self-sealing arrangement during molding and curing to prevent the forming gas from either migrating around the thermoplastic to force the molten plastic away from the mold cavity surface used to form the finished surface or escaping across the parting line of mold sections and outwardly from the mold cavity.
A yet further object of this invention is provision of fluid inlets which are efficient to provide a uniform gas pressure across the inner surface of the injected thermoplastic and are less costly than conventional nozzles and injection valves.
A further object of this invention is provision of a gas recirculation arrangement that enhances the cooling of the part.